Lipopolysaccharide (LPS) in the cell wall of theP. aeruginosais a major factor in the pathogenicity and virulence of this organism. Immunotype-specific serum antibodies to this LPS antigen (ALPS) are usually elevated with the onset of the bacteremia, act as opsonins, are protective in high levels, and are significantly associated with improved survival. In the present study, the ability of 11 patients with AIDS to mount a specific ALPS response with the onset ofP. aeruginosabacteria was evaluated prospectively. Of the 11 patients with AIDS only one had a substantial ALPS response, six mounted only a marginal ALPS response, and four had no ALPS response to the infecting strain of theP. aeruginosa.These data suggest most patients with AIDS do not exhibit a marked antigen-specific humoral response at the onset ofP. aeruginosabacteremia; this has important prognostic and therapeutic significance, as high ALPS titers are associated with survival.